A patient with normal vision is able to appropriately change the shape of the natural lens so as to focus an image upon the retina, and particularly upon the retinal macula. Myopic persons are unable to focus upon distant objects because the image thereof is formed a short distance (often measured in fractions of a millimeter) in front of the macula. There are various causes of myopia, among which may be listed the failure of the natural lens to change shape sufficiently, abnormal corneal curvature, and an abnormally long eye globe measured in the direction of the optical axis.
Eyeglasses commonly are employed to correct myopia of not greater than several diopters. Contact lenses can be used to correct myopia to the extent of perhaps twenty diopters. A surgical procedure known as a radial keratotomy has been employed in recent years to at least partially correct myopia, the procedure involving making radial, partial-thickness incisions in the cornea for the purpose of slightly flattening the cornea. Another surgical procedure, keratomilieusis, involves removal of a partial thickness corneal button, freezing and grinding the inner-side of the button to produce increased or decreased concavity, and then reattaching the corneal button, this procedure also tending to flatten the cornea. The surgical procedures thus described depend greatly upon the technical expertise and manual dexterity of the surgeon, and may in some cases lead to scarring of the cornea or to unintentional invasion of the anterior chamber with the consequent danger of infection. Further, a great deal of judgment must be used with respect to the depth, placement and number of radial incisions in the radial keratotomy procedure and in the degree of tissue removal in the keratomilieusis autograft procedure, and perfect results are seldom obtained. Trauma to the eye in the vicinity of the limbus, of course, may result in the onset of glaucoma.
The prostheses and surgical routines referred to above each have, as their object, a refractive correction anterior to the natural lens.
In the conditions known as progressive axial myopia and posterior staphyloma, the enlargement of the scleral and uveal coats of the eye results in stretching, thinning and deterioration of the retinal nerve tissue, which by its nature cannot grow. The interocular pressure in the globe itself tends to cause the globe to bulge out rearwardly. This, in turn, may cause structural changes in the retina leading to optically uncorrectible decreases in vision. In an effort to support the rearward portion of the globe against further distension and traction on the retina, scleral reinforcement techniques have been developed in which a band of scleral tissue is passed about the posterior portion of the globe to strengthen the globe. This procedure is particularly well reported in Frank B. Thompson, A Simplified Scleral Reinforcement Technique, AM. J. Opthalmol 86:782-790, 1978. In the reported procedure, a band of scleral tissue terminating in corneal portions and taken from a donor eye was passed posteriorly about the globe of an eye, the corneal end portions being sutured to the globe anteriorly of its equator. Although such slings by themselves may occasionally reduce an ectatic posterior staphyloma, the rearward portion of the globe is at best strengthened to prevent further distension and to limit the progression of posterior enlargement of the globe.